1. Field of the Invention
The invention relates to functional fluids having low Brookfield Viscosities comprising a mixture of base stocks and containing performance additives.
2. Description of the Related Art
Functional fluids comprise a broad range of lubricants that are used in automotive and industrial hydraulic systems, automotive transmissions, power steering systems, shock absorber fluids, and the like. These fluids transmit and control power in mechanical systems, and thus must have carefully controlled viscometric characteristics. In addition, these fluids may sometimes be formulated to provide multigrade performance so as to ensure year round operation in variable climates. Among the most important requirements for a functional fluid is low temperature fluidity, which can be measured by, for example, the Brookfield viscometer.
Automatic transmission fluids are one of the most common functional fluids, and an integral part of all automatic transmissions. Automatic transmissions are used in about 80% to 90% of all vehicles in North America and Japan and their use is becoming more commonplace in other parts of the world. They are the most complex and costly sub-assemblies of a vehicle and the major OEMs have stringent specifications to control all aspects of the components that go into their manufacture.
An automatic transmission comprises a torque converter, planetary gears, output drives and hydraulic system. The ATF acts as a hydraulic fluid to transfer power in the torque converter and to actuate complex controls to engage the gears to give the correct vehicle speed. The fluid must have the right viscometrics at ambient start-up temperatures, while maintaining sufficient viscosity at higher operating temperatures. ATF must also be very oxidation stable because it is subjected to high temperatures and is expected to remain in service for up to 100,000 miles in some cases.
Whereas in the past automatic transmission fluids generally used solvent neutral base stocks, and their use is still common in some applications, over the past few years, with the increasing performance demands being made on automatic transmission fluids, the use of hydrocracked base stocks have become more widespread. These base stocks tend to give improved low temperature performance and longer oxidation life.
It has now been found that particular blends of hydrocracked base stocks, which may also contain some minor amount of solvent neutral base stocks, give excellent low temperature Brookfield performance.